Immunoglobulin molecules may be one of five classes based on amino acid sequence of the constant region of the molecule. These classes are IgG, IgM, IgA, IgD and IgE. Each class has different biological roles based on class-specific properties (Roitt, I., Brostoff, J., Make, D. (2001), Immunology, 6th edition, pub. Mosby).
IgA is the first line of immune defense on mucosal surfaces where it is produced at approximately 3-5 g/day. Furthermore it is present in serum at around 3 mg/ml, and as such is the most abundantly produced immunoglobulin. One IgA monomer comprises two heavy chains and two light chains. Light chains may be of the kappa or lambda class, and are shared with other immunoglobulin classes. The heavy chains of IgA consist of an N-terminal variable region, followed by a constant region, a hinge region, and then two additional constant regions. At the C-terminus there is a tail-piece which has a function in dimerization of the molecule. IgA is found as two different isoforms, IgA1 and IgA2; the main difference in these isotypes is found in the hinge region, where IgA2 lacks a proline-rich region which in IgA1 is susceptible to bacterial protease activity. IgA1 is the predominant isotype in serum, whereas IgA2 is mainly produced by cells of the lamina propria where it is secreted on to mucosal surfaces. IgA2 is further divided into IgA(m)1 and IgA(m)2, depending on the nature of the disulphide linkage of heavy and light chains (Chintalacharuvu and Morrison, 1999).
Plasma IgA is found predominantly as a monomeric species, but mucosal IgA exists primarily in a dimeric form. A 15 kDa J-chain binds to the molecule, stabilizing the dimeric form, and upon secretion to the mucosal surface a 50-90 kDa secretory component becomes complexed to the molecule. This protects the IgA from degradation on the luminal surface (Johansen et al., 2000, 2001).
The IgA1 hinge region includes three to five O-linked glycans, and the heavy chain includes two N-linked glycans (Mattu et al., 1998; Novak et al., 2000). The IgA2 subclass contains an additional two or three N-linked glycans. The role these glycans play in protein stability or effector function is not clear.
IgG1 does not bind neutrophils with a high affinity, and therefore is not proficient in neutrophil activation. IgA exerts its biological effects by a variety of mechanisms (Dechant and Valerius, 2001). It binds to the FcαRI receptor (CD89), which is expressed on neutrophils, eosinophils, monocytes/macrophages, though not on NK cells. Activation of FcαRI causes responses such as the oxidative burst and degranulation, but is also important in the triggering of phagocytosis. Neutrophils are the most prevalent lymphocytes in the blood and when activated are a powerful tool to combat infection. A second mechanism by which IgA exerts a response is by directly binding to and neutralizing pathogens at mucosal surfaces, an effect enhanced by the dimeric nature of the IgA. Additionally, a mechanism by which antibodies are thought to eliminate target cells is by binding to, and often cross-linking, cell surface receptors (Ghetie et al., 1997; Tutt et al., 1998; Longo, 2002). This can lead to activation of signaling pathways resulting in arrest of cell growth or apoptosis. IgA does not appear to activate complement, but the other effector functions make it a potentially valuable adjunct to IgG in the clinical treatment of disease.
Few studies have been performed to test the value of IgAs as therapeutics (reviewed in Dechant and Valerius, 2001; Corthesy, 2002), in part because murine IgA does not bind the human FcαRI. Some studies have been performed which demonstrate IgA activity against infectious disease (Vidarsson et al., 2001; Spriel et al., 1999), and a few studies to look at cancer immunotherapy (Huls et al., 1999a; van Egmond et al., 2001).
There is also little data in the literature regarding IgA production in cell lines (reviewed in Chintalacharuvu and Morrison, 1999; Yoo et al., 2002). There is little information regarding productivity of such cells, but IgA production of up to 20 pg per cell per 24 hours has been described in Chinese Hamster Ovary (CHO) cells, though this included gene amplification (Berdoz et al., 1999). However, cells with high copy numbers of the transgene are reported to display instability upon propagation of the cells (Kim et al., 1998; Barnes et al., 2003). Glycan structures present on an IgA molecule produced in CHO cells has also been analyzed, and it was found that CHO cells produced predominantly tri-antennary structures, whereas serum IgA contains mostly bi-antennary structures (Mattu et al., 1998). This may reflect a difference between human cells and CHO cell culture.
A need remains for a good production platform for recombinant IgA production, without the drawbacks associated with the existing platforms.